[Haskell-beginners] doesFileExist cannot get ~/blah_blah right

Hi, I got "not exists ..." from fileExists <- doesFileExist "~/blah_blah" if fileExists then print "exists ..." else print "not exists ..." using the directory-1.1.0.2 package, when ~/blah_blah does exist. I do not want to change "~" to a specific home directory such as "/home/a_userid" for the sake of portability. What else can I do before the bug (suppose this is a bug) is fixed? Thanks, Hong